@book{Ref1,
   title = {Modern Control Engineering},
   author = {Katsuhiko Ogata},
   publisher = {Prantice Hall},
   year = {1997}
}
@book{Ref2,
   title = {Introduction to Modern Control Theory},
   author = {F. L. LEvis},
   publisher = {Prantice Hall},
   year = {1992}
   }
@book{Ref3,
   title = {Modern Control Systems},
   author = {Richard C. Doft, Davis && Robert H.
Bishop},
   publisher = {Prantice Hall},
   year = {2008}
   }
@book{Ref4,
   title = {Modern Control Engineering},
   author = {P.N. Paraskevopoulos},
   publisher = {Marcel Dacker},
   year = {2002}
   }
@book{Ref5,
   title = {Solving Control Engineering Problems with MATLAB},
   author = {Katsuhiko Ogata},
   publisher = {Marcel Dacker},
   year = {2002}
   }
@book{Ref6,
   title = {Airplane design},
   author = {Dr. Jan Roskam },
   publisher = {Darcorparation},
   year = {2000}
   }
@book{Ref7,
   title = {Elevator Traffic Handbook: Theory and Practice},
   author = {Gia Carol Barney},
   publisher = {Taylor & Francis},
   year = {2003}
   }
@UNPUBLISHED{Ref8,
   author = {Tang Bingxin},
   title = {Fundementals of Control Engineering},
   note = {Lecture notes},
   month = {jan},
   year = {1988}
   }
@ARTICLE{Ref9,
   AUTHOR  = {Neculai Andrei},
   TITLE   = {Modern Control Theory- Historical Perspective},
   YEAR    = {1993},
   JOURNAL = {jgr},
   VOLUME  = {v1},
   NUMBER  = {5},
   PAGES   = {1-12}
}
@book{Ref10,

This is BibTeX, Version 0.99c (Web2C 7.5.6)
The top-level auxiliary file: progressreport.aux
Case mismatch error between cite keys ref9 and Ref9
---line 27 of file progressreport.aux
 : \citation{ref9
 :               }
I'm skipping whatever remains of this command
The style file: chicago.bst
Database file #1: progressreport.bib
(There was 1 error message)

   title = {Linear Multivarible Control: A Geometric Approach},
   author = {W. Murray Wonham },
   publisher = {Springer},
   year = {1979}
   }